Model not smooth and imperfect after mirrored in Blender
It looks like you are using a "subdivision" modifier before the "mirror" modifier. That causes the rounding of the border you see in the lips.

Try to put the mirror modifier first (i.e. at the top of the list, use the triangle buttons to move a modifier). 

And what I usually also do in this kind of situations, is set the "clipping" option in the mirror modifier, select all vertices in the mirror plane and move them back and forth a bit to "lock" them on the mirror plane (a small movement along the mirror axis shows you very quickly when the vertices "lock").